1. Field of the Invention
This invention relates to a photonic crystal fibre, to a method of making a photonic crystal fibre and to a method of transmitting light along a photonic crystal fibre.
2. Discussion of Prior Art
Optical fibres are used for transmitting light from one place to another. Normally, optical fibres are made of more than one material. A first material is used to form a central light-carrying part of the fibre known as the core, while a second material surrounds the first material and forms a part of the fibre known as the cladding. Light can become trapped within the core by total internal reflection at the core/cladding interface. Total internal reflection in general causes no losses other than the intrinsic absorptive and scattering losses associated with the materials themselves. Conventional and commercial low-loss optical fibres typically have a total internal reflection structure; however, one limitation of the waveguiding mechanism (which we refer to as “index guiding”) is that the refractive index of the material forming the core must be higher than that of the material forming the cladding, in order to achieve total internal reflection. Even if the cladding is air (with a refractive index of approximately unity) the core material must still be a solid material for the fibre to be useful. In practice, using air as the cladding material is normally not advisable, because it does not offer sufficient mechanical or optical protection of the waveguiding core. Thus, conventional optical fibres consist of a solid or liquid core material surrounded by a solid cladding.
An optical fibre waveguide having a significantly different structure from that of conventional optical fibres has been demonstrated, in which a single microstructured material is used to form the fibre core and cladding. The introduction of morphological microstructure into the fibre—typically in the form of an array of small holes which run down the length of the fibre—alters the local optical properties of the fibre, making it possible to design and fabricate intricate waveguiding structures with most unusual properties. Such a fibre is an example of a photonic crystal fibre.
In one type of photonic crystal fibre, a fibre with a periodic array of air holes in its cross-section and with a single missing air hole in the centre (a “defect” in the crystal structure) forms a low-loss all-silica optical waveguide which remains monomode for all wavelengths within the transmission window of the silica. The waveguiding mechanism in that case is closely related to that in conventional optical fibres and is a form of total internal reflection from a material which has a lower apparent refractive index than that of pure silica.
Another type of waveguiding has also been demonstrated in a photonic crystal fibre with a periodic array of air holes. Light can become trapped in the vicinity of an extra air hole within the photonic crystal lattice (i.e. a “low-index” lattice defect), if the photonic crystal is appropriately designed to exhibit a “photonic band gap”. A photonic band gap is a range of parameters—for example, a range of frequencies or wavevectors—for which light would normally be expected to propagate in the cladding material but where there are no propagating modes due to the detail of the microstructuring. In fibres of that type that have been demonstrated to date (see, for example, J. C. Knight, J. Broeng, T. A. Birks and P. St. J. Russell, “Photonic Band Gap Guidance in Optical Fibres”, Science 282 1476 (1998)), light, propagating along a fibre, is trapped in the vicinity of a low-index defect within a fibre with a honeycomb array of air holes, but nonetheless is strongly confined to the high-index phase of the microstructured material. The light is evanescent in the air, so the observed guided mode is concentrated in the silica surrounding the extra air hole at the core.
Guidance of light through hollow fibres in the form of glass capillaries has been demonstrated, but such devices are inherently very leaky.